Electron gun device of field emission type

ABSTRACT

In an electron gun device of field emission type having a cathode tip, a first anode and a second anode, both surfaces of the first and second anodes face each other and are shaped to produce a lens electric field which acts to focus electrons emitted from the cathode tip in a space therebetween, whereby a finely focused electron beam can be obtained.

nited States Patent Tonomura et a1.

ELECTRON GUN DEVICE OF FIELD EMISSION TYPE lnventors: Akira Tonomura, Kodaira; Tadashi Ohtaka, Katsuta. both of Japan Assignee: Hitachi, Ltd., Tokyo, Japan Filed: Apr. 12, 1972 Appl. No.: 243,287

Foreign Application Priority Data Apr. 12, 1971 Japan 46-22336 US. Cl. 315/14, 315/31 R, 313/82 BF, 313/86, 335/210 Int. Cl. 1-101j 29/56 Field of Search 315/14, 10, 30, 31 R; 313/85 Q, 86, 82 BF; 250/49.5 D; 335/209, 210, 250

References Cited UNITED STATES PATENTS 4/1964 Koziak 313/82 R 1 Jan. 28, 1975 3,141,993 7/1964 Hahn 313/85 R X 3,225,248 12/1965 Scheffels. 315/31 R 3,585,546 6/1971 Yanaka 250/49.5 D X 3,597,608 8/1971 Gutter 250/49.5 D X 3,673,528 6/1972 Hughes 250/49.5 D X Primary Examiner-Benjamin R. Padgett Assisram Examiner-P. A. Nelson Attorney, Agent, or Firm-Craig & Antonelli [57] ABSTRACT In an electron gun device of field emission type having a cathode tip, a first anode and a second anode, both surfaces of the first and second anodes face each other and are shaped to produce a lens electric field which acts to focus electrons emitted from the cathode tip in a space therebetween, whereby a finely focused electron beam can be obtained.

7 Claims, 4 Drawing Figures 1 s1 VOLTAGE SOURCE 2 nd -5 VOLTAGE SOURCE 2 PATENTED 3,863,095

SHEET 10F 2 n FIG. I I

,' I si VOLTAGE L SOURCE 2nd N I sf FIG; 2 LQI v Y VOLTAGE 6 SOURCE 2nd \5 VOLTAGE SOURCE FIG. 4

PATENTED 3, 863 .095

sum 2 or 2 FIG. 5

1 ELECTRON GUN DEVICE OF FIELD EMISSION TYPE BACKGROUND OF THE INVENTION The present invention relates to an electron gun device of the field emission type, which is particularly suitable for an electron source in a scanning type electron microscope and the like.

DESCRIPTION OF THE PRIOR ART As is well known, a conventional electron gun device of the thermal emission type is so constructed as to emit thermal electrons from an oxide cathode heated by a filament. Usually, such an electron gun device has the disadvantages that it is difficult to use as an electron source in a scanning type electron microscope requiring a finely focused electron beam because of enlargement of the electron beam spot, when the accelerating voltage of the electron beam is raised to about IOKV.

Further, recently, the surface condition of a layer very near the surface of a specimen has been observed by scanning it with a low speed electron beam having a fine spot. In this case, it is impossible to analyze the surface if the intensity of the electron beam is not much larger than that in a conventional device. Namely, it is necessary to provide an electron beam having an intensity of lOO-l ,000 times that of the conventional device in order to satisfy such a requirement that the spot of the electron beam is less than 1M) and, the density thereof must be increased enough to enable detection of secondary electrons etc.

SUMMARY OF THE INVENTION A prime object of the present invention is to provide an electron gun device of the field emission type capable of producing a very finely focused electron beam.

Another object of the present invention is to provide an electron gun device of the field emission type capable of producing a low speed electron beam having very high intensity.

A further object of the present invention is to provide an electron gun device of the field emission type having a very simple construction.

In order to realize the above objects, the invention is characterized by employment of first and second anodes, having the surfaces thereof which face each other so shaped as to produce a lens electric field which acts to focus an electron beam emitted from a cathode tip by the first anode, in a space therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are schematic diagrams showing embodiments of the invention,

FIG. 3 shows curves representing both surfaces of the first and second anodes facing each other in said embodiment, and

FIG. 4 is a curve showing the potential variation with respect to an electron beam path in said embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. I, an electron gun device of the field emission type has a cathode tip I of needle shape, a first anode 2, a second anode 3, a first voltage source 4 for applying a voltage I01, to the first anode 2 and a second voltage source 5 for applying a voltage 101 2 to the second anode 3.

Both surfaces of the first and second anodes facing each other are so shaped as to produce a lens electric field which acts to focus an electron beam passing through the hole 6 of the first anode 2, in a space therebetween.

For example, both surfaces are designed to substantially coincide with equipotential surfaces defined in the following equation, respectively,

where a is a constant determined as described below and z is a coordinate on an axis between the centers of both surfaces of the first and second anodes.

In the above device, when a voltage I of about 2-3 KV and a voltage D of about 10-20 kV are applied to the first and second anodes respectively, a strong electric field is produced adjacent the top portion of the cathode tip 1, thereby causing electrons to be emitted from cathode tip 1 toward the first anode 2. The electron beam passing through the hole 6 of the first anode 2 is focused by means of the lens electric field and, thus, the finely focused electron beam, whose spherical aberration is small and intensity is large, can be obtained from the hole 7 of the second anode 3. The voltages D, and 9 are determined to satisfy the relationship I D as shown by a solid line in FIG. 4 and, as a result of this, the accelerated electron beam passing through the hole 6 is further accelerated as well as focused so that the electron beam obtained from the hole 7 has a high velocity.

On the other hand, it has recently been required to obtain a low speed electron beam having high intensity whose accelerating voltage is less than a few KV, for various purposes. Accordingly, it is necessary that the accelerating voltage of the electron beam is lower than that in the above device, but, since the voltage 101 1 must be within range between 2 and 3 KV for control of the amount of the emitted electrons, this voltage is not caused to be lower than this range. Therefore, the voltage 101 is lower than the voltage I in order to decelerate the electron beam in the space between the first and second a nodes as shown by a dotted line in FIG. 4.

In this case, the lens electric field produced in the space acts to focus the electrons similarly to the aforementioned case which diverge in the neighborhood of the first anode 2 and converge adjacent the second anode 3. In this manner, the low speed electron beam whose accelerating voltage is less than that applied to the first anode 2, can be obtained.

More particularly, the low speed electron beam having minimum aberration and high density can be obtained by employing the first and second anodes both surfaces of which face each other and are so shaped as to coincide with a particular figure as follows.

The following Table I shows experimental values of a spherical aberration coefficient C, and a chromatic aberration coefficient C with respect to each value of the constant a in the equation (I) described above.

TABLE I-Continued a C,(cm) C,(cm)

0.0 I35 2.8 0.1 107 2.7 0.2 9! 2 6 0.3 86 2 6 0.4 93 2 7 0.5 l l l 2 8 The spherical and chromatic aberration coefficients in Table I have been observed under conditions where voltages D and D are about 2,000 V and 200 V, respectively, the distance L between the cathode tip and the first anode is about 1 cm, and the distance 1 between both anodes is about 2 cm.

As is apparent from Table I, when a 0.3, the values of the spherical and chromatic aberration coefficients C, and C, are minimum, and at this time, the aforementioned equation may be represented as follows:

where r is a coordinate on an axis perpendicular to the z axis.

FIG. 3 shows shapes of both surfaces of the first and second anodes facing each other designed to substantially coincide with the equipotential surface defined by this equation.

The intensity of the electron beam depends upon the shape of the anode surface, particularly upon the spherical aberration determined in accordance with the shape. Namely, the radius d of the electron beam spot is represented in the following equation,

where a is the opening angle of the electron beam, and thus the intensity is proportional to the square of the radius d. In this way, since said intensity is remarkably reduced by changing the shape of the anode, in the design of the anode shape, the constant a may be determined to be within range between about 0.1 and 0.4 in which the intensity is reduced by about 25 percent.

In the practical production of the anode, since it may be difficult in practice from the standpoint of manufacturing time and cast to shape the anode to exactly coincide with the equipotential surface, the anode surface may be formed in such a manner as to coincide with a surface approximated with a plurality of straight line segments and arcs to represent the equipotential surface without substantial loss of advantage.

In addition, as shown in FIG. 2, the second voltage source may be connected between the first and second anodes and its output voltage is determined to be equal to the potential difference 1 D What we claim is:

1. An electron gum device of field emission type comprising:

a cathode tip of needle shape;

a first anode disposed downstream of said cathode tip in the beam path of electrons to be emitted from said cathode tip;

first voltage source means for applying a potential difference between said cathode tip and said first anode so as to produce an electric field for electron emission;

a second anode disposed downstream of said first anode in the beam path of electrons to be emitted from said cathode tip; and

second voltage source means for applying a potential difference between said first and second anodes;

wherein each of said first and second anodes has an aperture therein through which said electrons pass and wherein both surfaces of said first and second anodes facing each other are convex from a respective aperture toward the outer surfaces of a respective anode so as to produce a lens electric field which acts to focus said electrons. in a space therebetween, whereby a finely focused electron beam can be obtained.

2. An electron gun device according to claim 1, wherein said second voltage source means provides a voltage of such magnitude that the polarity of the potential difference between the first and second anodes is positive so as to accelerate said electrons in the space therebetween.

3. An electron gun device according to claim 1, wherein said second voltage source means provides a voltage of a magnitude such that the polarity of the potential difference between the first and second anodes is negative so as to decelerate electrons in the space therebetween.

4. An electron gun device of field emission type comprising:

a cathode tip of needle shape;

a first anode for causing electrons to be emitted from said cathode tip toward it;

said first anode being located farther downstream from said cathode tip in the beam path of said electrons;

first voltage source means for applying a potential difference between said cathode tip and said first anode so as to produce an electric field for electron emission;

a second anode located farther downstream from said first anode in the beam path of said electrons; and

second voltage source means for applying a potential difference between said first and second anodes;

wherein both surfaces of said first and second anodes facing each other are so shaped as to substantially coincide with the equipotential surface defined in the following equation, respectively, so as to provide a lens electric field which acts to focus said electrons, in a space therebetween, whereby a finely focused electron beam can be obtained:

1 (Z) 1 DJ/11.0 (1.5 a) Z (20 0.5)z"-l-az where D is the voltage applied to said first anode,

D is the voltage applied to said second anode,

a is a constant, and z is a coordinate on axis between the centers of both surfaces of the first and second anodes.

5. An electron gun device according to claim 4, wherein said constant is determined to be within range between about 0.1 and 0.4.

6. An electron gun device according to claim 4, wherein both surfaces of said first and second anodes wherein the distance between the cathode tip and said first anode is about 1 cm, the distance between said anodes is about 2cm, and the value of said constant a is about 0.3. 

1. An electron gum device of field emission type comprising: a cathode tip of needle shape; a first anode disposed downstream of said cathode tip in the beam path of electrons to be emitted from said cathode tip; first voltage source means for applying a potential difference between said cathode tip and said first anode so as to produce an electric field for electron emission; a second anode disposed downstream of said first anode in the beam path of electrons to be emitted from said cathode tip; and second voltage source means for applying a potential difference between said first and second anodes; wherein each of said first and second anodes has an aperture therein through which said electrons pass and wherein both surfaces of said first and second anodes facing each other are convex from a respective aperture toward the outer surfaces of a respective anode so as to produce a lens electric field which acts to focus said electrons, in a space therebetween, whereby a finely focused electron beam can be obtained.
 2. An electron gun device according to claim 1, wherein said second voltage source means provides a voltage of such magnitude that the polarity of the potential difference between the first and second anodes is positive so as to accelerate said electrons in the space therebetween.
 3. An electron gun device according to claim 1, wherein said second voltage source means provides a voltage of a magnitude such that the polarity of the potential difference between the first and second anodes is negative so as to decelerate electrons in the space therebetween.
 4. An electron gun device of field emission type comprising: a cathode tip of needle shape; a first anode for causing electrons to be emitted from said cathode tip toward it; said first anode being located farther downstream from said cathode tip in the bEam path of said electrons; first voltage source means for applying a potential difference between said cathode tip and said first anode so as to produce an electric field for electron emission; a second anode located farther downstream from said first anode in the beam path of said electrons; and second voltage source means for applying a potential difference between said first and second anodes; wherein both surfaces of said first and second anodes facing each other are so shaped as to substantially coincide with the equipotential surface defined in the following equation, respectively, so as to provide a lens electric field which acts to focus said electrons, in a space therebetween, whereby a finely focused electron beam can be obtained: Phi (z) Phi 1 +(( Phi 2- Phi 1)/) 1.0 + (1.5 + a) z - (2a + 0.5)z3+ az5 where Phi 1 is the voltage applied to said first anode, Phi 2 is the voltage applied to said second anode, a is a constant, and z is a coordinate on axis between the centers of both surfaces of the first and second anodes.
 5. An electron gun device according to claim 4, wherein said constant is determined to be within range between about 0.1 and 0.4.
 6. An electron gun device according to claim 4, wherein both surfaces of said first and second anodes facing each other are so shaped as to coincide with a surface approximated with a plurality of straight line segments and arcs to represent said equipotential surface.
 7. An electron gun device according to claim 4, wherein the distance between the cathode tip and said first anode is about 1 cm, the distance between said anodes is about 2cm, and the value of said constant a is about 0.3. 